1. Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to an image processing technique employable to process an index color image.
2. Description of the Related Art
The index color image is a bitmap image having a format capable of expressing an image with a color value defined in a color lookup table, instead of directly designating a color value for each pixel to express the image. The substance of image data constituting an index color image (i.e., a source image) is characteristic in that each pixel is defined by a reference number designating a color value defined in the color lookup table, not the color value itself. In a case where the number of colors included in an image is smaller, using an index color image is useful to reduce the amount of data while maintaining the resolution appropriately. Recently, index color images have been frequently used to satisfy various requirements with respect to color and resolution. As a method for speedily rendering an index color image, it is conventionally known to share the same color lookup table among a plurality of index color images as discussed in Japanese Patent Application Laid-Open No. 2004-25572.
Frequent usage of index color images possibly induces an increase of unnecessary expression of an image using an index color image, such as a simple image in which all of color values defined in the color lookup table is the same color.